
where \( (\cdot ,\cdot )_{\gamma ,2} \)  means the interpolation space defined by the
\( K \) -method (cf. [1]}). For convenience, the spaces \({}_0H^\gamma (a,b;\mathbb {C}) \)  and \( {}^0H^\gamma (a,b;\mathbb {C}) \)  will be
abbreviated to \( {}_0H^\gamma (a,b) \)  and \( {}^0H^\gamma (a,b) \) , respectively.
